This invention relates to an optical head device for use in data recording and retrieval systems, and in particular to a tracking and focus actuator for moving an objective lens.
Optical heads produce a focused beam of light on a medium containing information and detect the light from the medium to determine the information content of the medium. At the same time, a two-dimensional optical positioning mechanism or actuator is also incorporated in the optical heads to place and focus a light beam within a specific track on a rotating piece of optical media. The tracks or grooves on the optical media can be continuous or concentric in nature. An arm is used to move the optical head to the proper track, with the actuator focusing the light beam and compensating for small variations of the track without requiring the whole arm to be moved. The main task of the actuator is to keep the in the center of the groove and to provide a focused spot with a diameter determined by the numerical aperture of an objective lens which focuses the beam on the media.
Many actuators move only the objective lens and a portion of the optics relative to the rest of the optical head. A defocused spot will reduce the power density of the laser beam, which is required to provide the proper localized heating during the writing process. It also reduces the signal contrast during reading. Tracking in the center of the groove is the other feature that is of importance. If the beam is not in the center of the groove during the write operation, the read operation might not detect any information at all. With this in mind, it is imperative that the laser beam be properly focused and positioned during reading and writing operations.
This is complicated by the fact that the rotating medium has a certain amount of axial and radial run-out. The actuator must have the capability to follow these cyclic excursions with a frequency ranging from 8 Hz to 60 Hz. In addition, when the system moves the optical head across many tracks, the focus must be maintained so that the system can count the number of track crossings. Also, in executing a track jump, the system must accelerate the actuator to some speed and then de-accelerate when the proper track location is reached. These movements instill forces on the actuator which generally displaces the tracking motor in the actuator to its utter most extreme and thereby disables it. These requirements necessitate an actuator to have a large range of motion and sensitivity to follow media deviations and to have a position sensor for the tracking motor to maintain integrity during the times of track jumps.
Two-dimensional objective lens positioning devices (actuators, hereafter) must ideally be compliant in two modes (i.e. tracking and focus motions) but noncompliant in all other modes. Additional modes may result from deviations caused by harmonic resonances or other mechanical play in the actuator. These resonances vibrate the objective lens, thereby degrading the reading and writing operations.
Typical actuators use a variety of suspension means. One type uses a slide on a post (see FIG. 1) and is described in U.S. Pat. No. 4,482,988. The two-dimensions are achieved by sliding an objective lens 3 up and down on a post 2 and rotating the lens about the post in the direction of arrows b. Rotational movement is caused by the interaction of coil 8 with magnet 9, while vertical movement is caused by the interaction of coil 5 with magnet 7. Actuators of this type are generally low performance because friction on the post causes nonlinearity, the large clearance between the post and the sliding cylinder causes beam deviations, and lastly, because of the rotational nature of the actuator, it has limited travel without beam clipping.
Other types of actuators, such as the one shown in FIG. 2, uses a spring suspension that eliminates nonlinearity and makes a stable system. One drawback is that the actuator could be compliant in more than two modes. Harmonic distortions could be instilled in the suspensions at certain frequencies to cause optical misalignment. In this type of actuator the travel of the tracking motor is also limited by beam vignetting.
The actuator of FIG. 2 is described in U.S. Pat. No. 4,538,882. An objective lens 200 can be moved right and left for tracking by the interaction of coils 202 and 204 with magnets 206 and 208, respectively. For vertical, focusing movement, a coil 210 interacts with magnets 212 and 214. Two pairs of leaf springs 216 on one side and two pairs of leaf springs 218 on another side provide a rhomboidal parallel arrangement to support the objective lens and provide compliancy in only two directions.
Another type of actuator moves the objective lens up and down with the first coil surrounding the optical path to the objective lens and at least a pair of additional coils on the sides of the objective lens for side to side movement. Such systems are shown in U.S. Pat. Nos. 4,135,206; 4,193,091 and 4,437,177. In the last patent, U.S. Pat. No. 4,437,177, the entire optical head, including the laser and detector, is moved side to side.
Other actuator designs attempt to simplify and reduce the weight of the moving part of the actuator by using only two coils, with the laser beam passing through each of the coils. In U.S. Pat. No. 4,092,529, coils are positioned at the top and the bottom of a tube holding the objective lens, with each coil interacting with a magnet in a different orientation to provide movements in orthogonal directions. However, a four wire suspension system is used which does not provide noncompliance in the nonfocusing and nontracking directions. U.S. Pat. No. 4,557,564 shows a tube which has two criss-crossed coils which interact with a single magnet and are energized in phase for focus error correction and out of phase for tracking error correction. The tube is supported by two pairs of angled leaf springs which also do not provide total noncompliance in the nonfocus and nontracking directions.
All of the above discussed prior art, with the exception of Watabe U.S. Pat. No. 4,437,177, move the objective lens sideways relative to the laser beam for tracking, which will cause beam distortion for large movements. Watabe avoids this by moving the entire optical head, which requires more power because of the larger mass being moved.
Another method for avoiding beam clipping is shown in U.S. Pat. No. 4,135,083, which uses a fiber optic cable to couple the laser beam to the focus and tracking actuator.
A position sensor for the tracking motor is required in high performance actuator applications. A prior art version of a position sensor is shown in FIG. 3 and is discussed in U.S. Pat. No. 4,851,466. This common configuration for position sensing uses a LED 176 as a light source and a split detector 178a, 178b. The LED is separated from the detector by a flag 172 with a narrow slit 174. In a middle position of the flag, the amount of light sensed by each half of the detector is the same. As the flag moves with the tracking motor, the detected signals from the split detector will not be balanced. Thus, the actual position of the tracking motor can be monitored. The tracking range that can be detected is determined by the width of the split detector.